The nature of peptides that positively select T cells in the thymus remains poorly defined. To examine the role of peptides during positive selection in vivo, we generated transpenic mice expressing class II restricted TCR specific for analogs of a pigeon cytochrome C peptide PCC(43-58) presented by the Ab molecule. The TCR transgenic mice were backcrossed to mice simultaneously lacking H2-M, invariant chain and TCRa-chain, where development of transpenic thymocytes is arrested at the CD4+CD8+ stage, due to the lack of the endogenously selecting peptide(s). However, injection of the soluble agonist peptide could restore positive selection of these thymocytes, making this model the first in vivo system of peptide induced positive selection of T cells. Hence, the specific aims of this proposal are as follow: 1: To determine biological properties of soluble peptides that can initiate positive selection of CD4+ T cells using this in vivo system. 2: To identify neutral peptides that positively select CD4+ T cells. 3: To analyze the activation and survival requirements of CD4+ T cells expressing one particular abTCR, positively selected in different in vivo environments. 4: To compare gene and protein expression profiles in thymocytes shortly following the onset of positive selection.